The Pancreas:As chyme floods into the small intestine from the stomach, two things must happen: acid must be quickly and efficiently neutralized to prevent damage to the duodenal mucosa macromolecular nutrients - proteins, fats and starch - must be broken down much further before their constitutents can be absorbed through the mucosa into blood The pancreas plays a vital role in accomplishing both of these objectives, so vital in fact that insufficient exocrine secretion by the pancreas leads to starvation, even if the animal is consuming adequate quantities of high quality food. In addition to its role as an exocrine organ, the pancreas is also an endocrine organ and the major hormones it secretes - insulin and glucagon - play a vital role in carbohydrate and lipid metabolism. They are, for example, absolutely necessary for maintaining normal blood concentrations of glucose.----------------- Gross and Microscopic Anatomy of the PancreasThe pancreas is a elongated organ, light tan or pinkish in color, that lies in close proximity to the duodenum. It is covered with a very thin connective tissue capsule which extends inward as septa, partitioning the gland into lobules. The image below shows a canine pancreas in relation to the stomach and duodenum.----------------- The bulk of the pancreas is composed of pancreatic exocrine cells and their associated ducts. Embedded within this exocrine tissue are roughly one million small clusters of cells called the Islets of Langerhans, which are the endocrine cells of the pancreas and secrete insulin, glucagon and several other hormones. In the histologic image of an equine pancreas seen below, a single islet is seen in the middle as a large, pale-staining cluster of cells. All of the surrounding tissue is exocrine.----------------- Pancreatic exocrine cells are arranged in grape-like clusters called acini. The exocrine cells themselves are packed with membrane-bound secretory granules which contain digestive enzymes that are exocytosed into the lumen of the acinus. From there these secretions flow into larger and larger, intralobular ducts, which eventually coalesce into the main pancreatic duct which drains directly into the duodenum. The lumen of an acinus communicates directly with intralobular ducts, which coalesce into interlobular ducts and then into the major pancreatic duct. Epithelial cells of the the intralobular ducts actually project "back" into the lumen of the acinus, where they are called centroacinar cells. The anatomy of the main pancreatic duct varies among species. In some animals, two ducts enter the duodenum rather than a single duct. In some species, the main pancreatic duct fuses with the common bile duct just before its entry into the duodenum. Additional information on microscopic anatomy of the pancrease can be found in the section on Histology of the Pancreas.---------------------------------- Exocrine Secretions of the PancreasPancreatic juice is composed of two secretory products critical to proper digestion: digestive enzymes and bicarbonate. The enzymes are synthesized and secreted from the exocrine ascinar cells, whereas bicarbonate is secreted from the epithelial cells lining small pancreatic ducts. Digestive EnzymesThe pancreas secretes a magnificent battery of enzymes that collectively have the capacity to reduce virtually all digestible macromolecules into forms that are capable of, or nearly capable of being absorbed. Three major groups of enzymes are critical to efficient digestion: ProteasesDigestion of proteins is initiated by pepsin in the stomach, but the bulk of protein digestion is due to the pancreatic proteases. Several proteases are synthesized in the pancreas and secreted into the lumen of the small intestine. The two major pancreatic proteases are trypsin and chymotrypsin, which are synthesized and packaged into secretory vesicles as an the inactive proenzymes trypsinogen and chymotrypsinogen. As you might anticipate, proteases are rather dangerous enzymes to have in cells, and packaging of an inactive precursor is a way for the cells to safely handle these enzymes. The secretory vesicles also contain a trypsin inhibitor which serves as an additional safeguard should some of the trypsinogen be activated to trypsin; following exocytosis this inhibitor is diluted out and becomes ineffective - the pin is out of the grenade. Once trypsinogen and chymotrypsinogen are released into the lumen of the small intestine, they must be converted into their active forms in order to digest proteins. Trypsinogen is activated by the enzyme enterokinase, which is embedded in the intestinal mucosa. Once trypsin is formed it activates chymotrypsinogen, as well as additional molecules of trypsinogen. The net result is a rather explosive appearance of active protease once the pancreatic secretions reach the small intestine. ---------------------------------- Trypsin and chymotrypsin digest proteins into peptides and peptides into smaller peptides, but they cannot digest proteins and peptides to single amino acids. Some of the other proteases from the pancreas, for instance carboxypeptidase, have that ability, but the final digestion of peptides into amino acids is largely the effect of peptidases in small intestinal epithelial cells. More on this later. Pancreatic LipaseThe major form of dietary fat is triglyceride, or neutral lipid. A triglyceride molecule cannot be directly absorbed across the intestinal mucosa. Rather, it must first be digested into a 2-monoglyceride and two free fatty acids. The enzyme that performs this hydrolysis is pancreatic lipase, which is delivered into the lumen of the gut as a constituent of pancreatic juice. Sufficient quantities of bile salts must also be present in the lumen of the intestine in order for lipase to efficiently digest dietary triglyceride and for the resulting fatty acids and monoglyceride to be absorbed. This means that normal digestion and absorption of dietary fat is critically dependent on secretions from both the pancreas and liver. Pancreatic lipase has recently been in the limelight as a target for management of obesity. The drug orlistat (Xenical) is a pancreatic lipase inhibitor that interferes with digestion of triglyceride and thereby reduces absorption of dietary fat. Clinical trials support the contention that inhibiting lipase can lead to significant reductions in body weight in some patients. AmylaseThe major dietary carbohydrate for many species is starch, a storage form of glucose in plants. Amylase is the enzyme that hydrolyses starch to maltose (a glucose-glucose disaccharide), as well as the trisaccharide maltotriose and small branchpoints fragments called limit dextrins. The major source of amylase in all species is pancreatic secretions, although amylase is also present in saliva of some animals, including humans. Other Pancreatic EnzymesIn addition to the proteases, lipase and amylase, the pancreas produces a host of other digestive enzymes, including ribonuclease, deoxyribonuclease, gelatinase and elastase. Bicarbonate and WaterEpithelial cells in pancreatic ducts are the source of the bicarbonate and water secreted by the pancreas. The mechanism underlying bicarbonate secretion is essentially the same as for acid secretion parietal cells and is dependent on the enzyme carbonic anhydrase. In pancreatic duct cells, the bicarbonate is secreted into the lumen of the duct and hence into pancreatic juice.----------------- Carbonic AnhydrasesCarbonic anhydrases are enzymes that catalyze the hydration of carbon dioxide and the dehydration of bicarbonate: CO2 + H2O <-----> HCO3- + H+These carbonic anhydrase-driven reactions are of great importance in a number of tissues. Examples include: Parietal cells in the stomach secrete massive amounts of acid (i.e. hydrogen ions or protons) into the lumen and a corresponding amount of bicarbonate ion into blood.Pancreatic duct cells do essentially the opposite, with bicarbonate as their main secretory product.Secretion of hydrogen ions by the renal tubules is a critical mechanism for maintaining acid-base and fluid balance.Carbon dioxide generated by metabolism in all cells is removed from the body by red blood cells that convert most of it to bicarbonate for transport, then back to carbon dioxide to be exhaled from the lungs. Carbonic anhydrase isozymes are metalloenzymes consisting of a single polypeptide chain (Mr ~ 29,000) complexed to an atom of zinc. They are incredibly active catalysts, with a turnover rate (kcat) of about 106 reactions per second! Catalytic activity depends on ionization of a group of pKa 7 and, as you might expect from thinking about when and where the above reactions take place, the hydration reaction depends on the ionized group being in the basic form, and for dehydration in the acidic form. Carbonic anhydrase inhibitors have been used theraputically. The prototype of such drugs is acetazolamide, which is still sometimes used as a diuretic to treat certain edematous conditions and for therapy of some types of glaucoma. The discovery of this drug is actually an interesting story. It is a member of the sulfonamides, a group of antibacterial agents which, when intially investigated, were shown to induce a metabolic acidosis because they inhibited excretion of hydrogen ion from the kidney.---------------The Parietal Cell: Mechanism of Acid SecretionThe best-known component of gastric juice is hydrochloric acid, the secretory product of the parietal, or oxyntic cell. It is known that the capacity of the stomach to secrete HCl is almost linearly related to parietal cell numbers. When stimulated, parietal cells secrete HCl at a concentration of roughly 160 mM (equivalent to a pH of 0.8). The acid is secreted into large cannaliculi, deep invaginations of the plasma membrane which are continuous with the lumen of the stomach.When acid secretion is stimulated there is a dramatic change in the morphology of the membranes of the parietal cell. Cytoplasmic tubulovesicular membranes which are abundant in the resting cell virtually disappear in concert with a large increase in the cannalicular membrane. It appears that the proton pump as well as potassium and chloride conductance channels initially reside on intracellular membranes and are transported to and fused into the cannalicular membrane just prior to acid secretion. The epithelium of the stomach is intrinsically resistant to the damaging effects of gastric acid and other insults. Nonetheless, excessive secretion of gastric acid is a major problem in human and, to a lesser extent, animal populations, leading to gastritis, gastric ulcers and peptic acid disease. As a consequence, the parietal cell and the mechanisms it uses to secrete acid have been studied extensively, leading to development of several drugs useful for suppressing acid secretion. Mechanism of Acid SecretionThe hydrogen ion concentration in parietal cell secretions is roughly 3 million fold higher than in blood, and chloride is secreted against both a concentration and electric gradient. Thus, the ability of the partietal cell to secrete acid is dependent on active transport. The key player in acid secretion is a H+/K+ ATPase or "proton pump" located in the cannalicular membrane. This ATPase is magnesium-dependent, and not inhibitable by ouabain. The current model for explaining acid secretion is as follows: Hydrogen ions are generated within the parietal cell from dissociation of water. The hydroxyl ions formed in this process rapidly combine with carbon dioxide to form bicarbonate ion, a reaction cataylzed by carbonic anhydrase.Bicarbonate is transported out of the basolateral membrane in exchange for chloride. The outflow of bicarbonate into blood results in a slight elevation of blood pH known as the "alkaline tide". This process serves to maintain intracellular pH in the parietal cell.Chloride and potassium ions are transported into the lumen of the cannaliculus by conductance channels, and such is necessary for secretion of acid.Hydrogen ion is pumped out of the cell, into the lumen, in exchange for potassium through the action of the proton pump; potassium is thus effectively recycled.Accumulation of osmotically-active hydrogen ion in the cannaliculus generates an osmotic gradient across the membrane that results in outward diffusion of water - the resulting gastric juice is 155 mM HCl and 15 mM KCl with a small amount of NaCl. These processes are depicted in the animation below. Control of Acid SecretionParietal cells bear receptors for three stimulators of acid secretion, reflecting a triumverate of neural, paracrine and endocrine control: Acetylcholine (muscarinic type receptor) Gastrin Histamine (H2 type receptor) Histamine from enterochromaffin-like cells may well be the primary modulator, but the magnitude of the stimulus appears to result from a complex additive or multiplicative interaction of signals of each type. For example, the low amounts of histamine released constantly from mast cells in the gastric mucosa only weakly stimulate acid secretion, and similarly for low levels of gastrin or acetylcholine. However, when low levels of each are present, acid secretion is strongly forced. Additionally, pharmacologic antagonists of each of these molecules can block acid secretion. Histamine's effect on the parietal cell is to activate adenylate cyclase, leading to elevation of intracellular cyclic AMP concentrations and activation of protein kinase A (PKA). One effect of PKA activation is phosphorylation of cytoskeletal proteins involved in transport of the H+/K+ ATPase from cytoplasm to plasma membrane. Binding of acetylcholine and gastrin both result in elevation of intracellular calcium concentrations. The animation below depicts acid secretion by the parietal cell. Even though many of the actors are unlabeled, you should be able to deduce the identity of all the components you see.-------------------- Control of Pancreatic Exocrine SecretionAs you might expect, secretion from the exocrine pancreas is regulated by both neural and endocrine controls. During interdigestive periods, very little secretion takes place, but as food enters the stomach and, a little later, chyme flows into the small intestine, pancreatic secretion is strongly stimulated. Like the stomach, the pancreas is innervated by the vagus nerve, which applies a low level stimulus to secretion in response to anticipation of a meal. However, the most important stimuli for pancreatic secretion comes from three hormones secreted by the enteric endocrine system: Cholecystokinin: This hormone is synthesized and secreted by enteric endocrine cells located in the duodenum. Its secretion is strongly stimulated by the presence of partially digested proteins and fats in the small intestine. As chyme floods into the small intestine, cholecystokinin is released into blood and binds to receptors on pancreatic acinar cells, ordering them to secrete large quantities of digestive enzymes. Secretin: This hormone is also a product of endocrinocytes located in the epithelium of the proximal small intestine. Secretin is secreted (!) in response to acid in the duodenum, which of course occurs when acid-laden chyme from the stomach flows through the pylorus. The predominant effect of secretin on the pancreas is to stimulate duct cells to secrete water and bicarbonate. As soon as this occurs, the enyzmes secreted by the acinar cells are flushed out of the pancreas, through the pancreatic duct into the duodenum. Gastrin: This hormone, which is very similar to cholecystokinin, is secreted in large amounts by the stomach in response to gastric distention and irritation. In addition to stimulating acid secretion by the parietal cell, gastrin stimulates pancreatic acinar cells to secrete digestive enzymes. Stop and think about this for a minute - control of pancreatic secretion makes perfect sense. Pancreatic secretions contain enzymes which are needed to digest proteins, starch and triglyceride. When these substances enter stomach, and especially the small intestine, they stimulate release of gastrin and cholecystokinin, which in turn stimulate secretion of the enzymes of destruction. Pancreatic secretions are also the major mechanism for neutralizing gastric acid in the small intestine. When acid enters the small gut, it stimulates secretin to be released, and the effect of this hormone is to stimulate secretion of lots of bicarbonate. As proteins and fats are digested and absorbed, and acid is neutralized, the stimuli for cholecystokinin and secretin secretion disappear and pancreatic secretion falls off.